This invention relates to an auxiliary steering control apparatus for use with a motor vehicle having controllable dynamic characteristics including yaw velocity, side velocity, roll velocity, etc. Although the invention will be described in connection with a rear wheel steering control system, it should be understood that the invention is equally applicable to other vehicle dynamic characteristic control systems including active suspension control systems and the like utilizing information on steering wheel position to control the dynamic characteristics of motor vehicles.
For example, U.S. patent application Ser. No. 07/375,629, filed July 6, 1989, discloses a rear wheel steering control apparatus which includes a hydraulic rear wheel steering actuator for controlling the angle of steering of the rear wheels based on the front wheel steering angle and the vehicle speed. The control apparatus calculates a target value for rear wheel steering angle in a manner to provide a first order advance phase inversion control where the rear wheels are turned for a moment in the direction opposite to the direction in which the front wheels are turned and then are turned in the same direction as the front wheel steering direction. When the rear wheels are turned in the direction opposite to the front wheel steering direction, a cornering force is applied in the yaw direction so as to decrease the time required for the yaw rate to increase to a sufficient extent. After a sufficient yaw rate is obtained, the rear wheels are turned in the same direction as the front wheel steering direction so as to limit the yaw rate increase and, thus, decrease the side slip angle. This is effective to increase the steering operation stability and decrease the steering operation response time.
The rear wheel steering control apparatus also includes a cutoff valve which is closed to interrupt the application of working pressure to the rear wheel steering actuator when a failure occurs in the vehicle speed sensor or the circuit associated therewith. After the cutoff valve is closed, the working pressure leaks through the closed cutoff valve to permit the rear wheel steering actuator to avoid a sudden rear wheel position change. If a failure occurs in the vehicle speed sensor or the circuit associated therewith when the rear wheels are steered in the direction opposite to the front wheel steering direction, however, the vehicle tends to spin due to its oversteer characteristic held until the working pressure to the rear wheel steering actuator decreases to zero even if the cutoff valve operates in order.